This invention relates to a hard-material coated wafer, a method of coating a substrate with hard material, a polishing apparatus and a polishing method. The hard-material wafer can be utilized for SAWs (surface acoustic wave devices), thermistors, substrates for semiconductor devices, protecting films of discs, and X-ray windows. Here xe2x80x9chard materialsxe2x80x9d generally indicate diamond, c-BN, or diamond-like carbon. All the hard materials cited herein have a Vickers hardness of more than Hv3000 in the state of bulk materials. The hard materials are endowed with high sound velocity which is determined by the ratio of Young modulus divided by density. Therefore, the velocity of the surface acoustic wave is extremely high. In particular, such hard material-coated wafers now attract attention as a material for the substrates of SAW devices. Suitable applications of SAWs include filters, phase shifters or convolvers. Diamond and c-BN which are intrinsically insulators can be converted to semiconductors by doping them with some impurity.
This invention further relates to a polishing method and a polishing apparatus for polishing surfaces of hard-material coated wafers which can be utilized in the technical fields of electronics, optics or optoelectronics. The hard-material coated wafer is referred to herein as a complex wafer. The wafer has a hard film which is made of a material selected from the group of diamond, diamond-like carbon and c-BN (cubic boron nitride), and a substrate base wafer. The base wafers are made of a softer material than the hard film. For example, Si or Mo is adopted as the base substrate. In particular, this invention is directed to a method and apparatus of polishing diamond-coated wafers which have been synthesized by the vapor phase CVD methods.
This application claims the priority of Japanese Patent Application No. 165914/1994 filed Jun. 24, 1994 and No. 133773/1994 filed May 23. The above-mentioned hard materials are favored with excellent physical and chemical properties. However, these materials have not been utilized in various fields for practical uses, since fabrication of wide and inexpensive plates or wafers of the materials has not been accomplished to date. Since the hard materials are provided with several physical and chemical advantages, actual applications of the hard materials to various objects are earnestly desired and have been attempted by applying the technology of silicon semiconductor devices to the hard materials. The object has been produce wide plates (or wafers) of the hard materials.
Technologies have already ripened into a definite level capable of producing films of diamond, c-BN (cubic boron nitride) or diamond-like carbon by vapor phase deposition methods. The vapor phase deposition method makes a hard-material of the material by supplying a material gas to a pertinent substrate heated at a suitable temperature, letting the gas react with the hot substrate and depositing a film of the hard material on the substrate in vapor phase. A film of diamond or c-BN is produced by introducing a material gas including hydrogen gas and hydrocarbon gas, or another material gas including hydrogen gas, boron-containing gas and nitrogen-containing gas in the reaction chamber, supplying the material gas on the heated substrate, synthesizing diamond or c-BN by chemical reaction and depositing the synthesized material as a film on the substrate.
There are some methods for exciting the material gas, including, for example, a hot filament CVD method, a microwave plasma CVD method, a radio wave plasma CVD method or a DC plasma jet CVD method. Some methods are capable of making a wide film of hard materials on a substrate. However, the speed of synthesis is so slow that the methods cannot easily make a thick film at present. In accordance with these methods, a long for deposition is needed to make a considerably thick film on the substrate.
Nevertheless, there are still no pure wafers consisting only of a hard material free from a substrate. In other words, at present, there are no diamond wafers or c-BN wafers in their true meanings, because prior technology has not been able to produce a pure diamond wafer or a pure c-BN wafer.
The application of the hard materials, i.e., diamond, c-BN or diamond-like carbon to electronics technology requires wide area wafers of the hard materials. While surface acoustic wave devices on very small diamond substrates have been produced, larger substrates have not.
However the fact that a new device was fabricated on a quite small diamond substrate, e.g., from 5 mm square to 10 mm square, was rather insignificant from an industrial standpoint, even if the device itself exhibited an excellent performance. Since the small substrate allowed only a small number of devices to be made on it, the productivity was poor. The devices made on the small substrate had little practical significance due to the poor productivity.
What brought about the success of silicon semiconductor devices is the ability to treat a wide area Si wafer by the same wafer processes at the same time and make several equivalent devices in a short time. It has been believed that the same would perhaps hold for the hard materials. If diamond, c-BN or diamond-like carbon is to obtain a practical importance as substrates, the material should be formed into wide, round thin plates (wafers). The advent of the wide wafers will enable manufacturers to apply the technology which has been developed by the silicon semiconductor industry to the hard materials.
In the case of silicon, big single crystals with a wide section can easily be grown by Czochralski methods, and mainly 8-inch wafers are produced for making devices at present. 12-inch wafers also can be produced now for silicon.
However, a diamond or c-BN single crystal cannot be grown by the conventional methods, e.g., Czochralski methods, at present. Thus, it is still not promising to produce homogeneous wafers consisting only of a single material of the hard materials, unlike silicon (Si) or gallium arsenide (GaAs). In fact, it is impossible to make wide, homogeneous diamond wafers or c-BN wafers by conventional methods.
This invention gives up the attempt of making a homogeneous wide wafer consisting only of a single material of diamond, c-BN or diamond-like carbon. Instead of starting from the premise of making a homogeneous bulk single crystal, this invention employs a substrate of a different material than the film for making complex wafers containing a non-hard material substrate and hard material films formed on the substrate. The present invention intends to make a hard material film on a commonplace material, e.g., Si, GaAs or so, which can easily be produced or obtained. The complex wafter having a substrate plus a hard material film provides the possibility of making a wide hard material wafer by employing a wide base wafer as a substrate. The base wafer of non-hard material plays the role of the base mount on which the hard material film is deposited. The film on the base wafer is the principal portion of the wafer which will contribute to the production of semiconductor devices or SAWs.
A homogeneous wafer consisting only of pure diamond or c-BN, made by forming a very thick film on the substrate and eliminating the substrate by etching, is still unpractical, because it takes very long time and very much material to deposit such a thick layer. Further, a large inner stress would break the film when the substrate is etched away. Therefore, the production of a freestanding film remains an unpractical object.
This invention is directed to the complex, non-homogeneous wafer having a non-hard material wafer as the substrate of the wafer. The substrate of the wafer does not cause a problem in application, since almost all the devices make use only of the surface of the wafer. This invention employs a non-homogeneous, complex wafer having a Si or GaAs surface on the bottom and a diamond or c-BN surface on the top. The adoption of the complex wafer can overcome the difficulty of making a wide bulk single crystal of the hard materials, because complex wafers can be made by the thin film-formation technology. The term xe2x80x9chard material waferxe2x80x9d in the present invention is quite different from the ordinary wafers produced by slicing an ingot of bulk single crystal. The wafers proposed by the present invention are different from the silicon wafers or the gallium arsenide wafers in their production methods.
This invention satisfies another requirement of the hard material wafers. As mentioned before, it has been attempted to make surface acoustic wave devices on a diamond substrate of 3 mm square or 5 mm square. Such attempts may have a certain significance from the standpoint of technological research. However, such a small substrate is useless for making devices on the industrial scale. The industrial production requires circularity, constant thickness, flatness, unbentness, and large diameter for substrate wafers.
With regard to the area of a wafer, the application to electronics devices demands that the materials have wide circular or rectangular wafers of at least an inch diameter (25 mm). Wafers of a diameter less than an inch cannot be treated by the wafer processes which have been developed by the silicon semiconductor industries. Furthermore, a two-inch diameter wafer is better than a one-inch diameter wafer. The three-inch diameter wafer is more preferable to the two-inch diameter wafer from the standpoint of industrial production, which desires large, flat and smooth wafers.
Fortunately, progress in the technology of vapor phase synthesis enables the production of a considerably broad film of diamond or c-BN on a suitable substrate. However, the blunt possibility of forming wide thin films does not increase the probability of making wide wafers suitable for the wafer process. Wafers having rugged surfaces are useless. Namely, a smooth surface is one of the requisites for wafers. Flatness or unbentness is another matter of significance. The hard material wafer must be a mirror wafer without bending or bow. Here flatness means a long range regularity of a surface. Bending or bow indicates a long range irregularity of a surface. Smoothness is defined as a short range regularity, and ruggedness or raggedness signifies a short range irregularity with may convex or concave imperfections on a surface.
The wafer must be mirror-smooth and flat in order to make devices on the wafer by photolithography. If the wafer is not flat or not mirror-smooth, exact patterns cannot be drawn on the wafer by an optical means.
Films which have been made by the vapor phase deposition have a lot of micro convexes or concaves on their surfaces. Wavy morphology is sometimes formed on the surface of the film. In other cases, granular convexes distribute on the surface of the film. In general, the films which have been made by vapor phase deposition cannot be used as a substrate wafer due to the ruggedness of the surface.
If a wafer suffers from a rugged surface, the wafer could be converted into a mirror wafer by eliminating the ruggedness out of the surface with a polishing apparatus. The polishing would remove the convexes, concaves or wavy morphology out of the surface and would make a flat, smooth wafer. Actually in the case of silicon wafers, mirror wafers are made by slicing an Si ingot into a lot of as-cut wafers, etching the as-cut wafers and polishing the etched wafers by a polishing apparatus. Can the same treatment make flat, smooth hard-material coated wafers? No, the same treatment does not make smooth wafers. The polishing of the hard materials has presented unexpected problems.
Since the convex or concave imperfection would be removed from the surface of the hard-material coated wafer by polishing, the starting film must have a sufficient thickness to allow for a margin of polishing. It takes a long time and much material to produce a thick film of the hard materials. This has obvious economic disadvantages.
Another big problem is the difficulty of polishing of the hard materials. The hard-material film can be to difficult to polish. Diamond and c-BN are the hardest materials which are far more difficult to polish than silicon wafers. Diamond or c-BN is polished by diamond powder as an abrasive. The polishing is sometimes called the xe2x80x9ctogether-polishingxe2x80x9d, because diamond powder shaves diamond by reducing itself during polishing. Lengthy together-polishing can polish surfaces of diamond crystals. The problem of hardness thus can be solved by practicing together-polishing.
However, there is still a more difficult problem. The thermal expansion coefficients are different between the hard material-coated film and the non-hard material substrate. The complex wafer will have a large inner stress due to the difference between the thermal expansion coefficients of the film and substrate. After the complex wafer is produced in a reaction chamber, and is cooled, the complex wafer will bend due to the release of the inner stress when it is removed from of the reaction chamber.
In one case, the complex wafer bends convexly to the side of the film. In another case, the complex wafer bends concavely to the side of the substrate. In yet another case, the wafer stays flat as a whole. The directions of bending are now defined with regard to the side of the film. The wafer bending is classified by the directions of bending. The bending which defines a convex shape on the surface of the film is called xe2x80x9cconvex-distortionxe2x80x9d. The bending which defines a concave shape on the surface of the film is called xe2x80x9cconcave-distortionxe2x80x9d. The problem of the distortion is still latent in the case of small plates. No distortion occurs for small complex plates of, e.g., 3 mm square or 5 mm square.
The present invention aims at making complex wafers of 1 inch diameter, 2 inch diameter or still bigger diameters. Large amounts of bending appears in the complex wafer due to the difference of the thermal expansion coefficients or the inner stress of the complex film itself. The broadness of the wafer induces a large distortion. The problem of the distortion is quite serious for the hard material coating wafers. In the case of silicon, large, flat mirror wafers can easily be produced since Si wafers are homogeneous without the complexity of a multilayer structure. However, the distortion causes a significant problem due to the non-homogeneity and complexity in the case of hard material-coated wafers.
The distortion of wafers causes many difficulties. The distortion prevents the photolithograpy from transcribing mask patterns exactly on the resist coating of the wafer. This is a big drawback in itself. To begin with, the distortion of a wafer is a large hindrance for polishing the wafer. A conventional polishing machine cannot polish a bending wafer at all. Polishing converts a rugged wafer into a mirror wafer. If polishing is impossible, no mirror wafer can be produced. If a wafer is not mirror-polished, the photolithography is entirely impotent. Thus, it is impossible to make devices on a rugged wafer. Polishing is a fundamental requirement for wafers.
Diamond is the hardest material; there is no harder material than diamond. Thus, diamond is polished by diamond. Namely, diamond plates are polished by an apparatus using diamond powder.
There are two methods for polishing diamond plates. One method adopts free polishing powder. The other adopts fixed polishing powder. Skife polishing which has been known as a way of the former polishing method uses an iron polishing plate and diamond free polishing powder. The method polishes a diamond plate by the steps of gluing the diamond plate to a holder, pushing the holder on a rotary polishing turn-table (round whetstone), supplying polishing liquid containing diamond powder, revolving the turn-table, rotating the holder and whetting the object diamond plate by the physical action of diamond granules. The method has a drawback of producing a large amount of diamond powder waste since it depends on free diamond powder in polishing. The consumption of diamond powder raises the cost of whetting. Another fault of the method is the slow whetting speed and the poor accuracy of finishing.
Another (fixed powder) method grinds diamond plates with a whetstone on which diamond powder is fixed. There are several kinds of diamond whetstones, which are classified by the manner of fixing diamond granules on the whetstones. The whetstone on which diamond powder is fixed by phenol resin, polyimide resin or so is called a resin-bonded whetstone. Another whetstone on which diamond powder is fixed by bronze, cobalt, tungsten, iron, nickel and so forth is called a metal-bonded whetstone. The whetstone on which diamond powder is fixed by plating of nickel etc. is called an electrodeposition whetstone. These whetstones using fixed diamond powder have an advantage of avoiding the waste of diamond granules.
The fixed granule method polishes the surface of diamond plates by the steps of sticking a diamond plate on a holder, pushing the holder on a rotary whetstone table, revolving the whetstone table, rotating also the holder around own axis and polishing the surface of the diamond by the physical interaction with the fixed diamond granules.
All the methods grind diamond plates or diamond films by the physical contact with the diamond granules. The methods depend on the physical action of the diamond powder. Because of the heightened physical interaction, these methods require imposing a heavy load on the polishing faces. The high pressure of the load enables the diamond powder to scuff, scratch, or scrape the surface of the object diamond plates or films. The heavy load also defaces the diamond granules either being fixed on the whetstone or flowing in the liquid. Without the heavy load, the object diamond can be ground no more, slipping in vain on the whetstone.
It has been suggested to develop a polishing method which dispenses with the heavy load. Japanese Patent Laying Open No. 2-26900 (26900/90) proposes a thermochemical method which polishes diamond by chemical reaction by bringing diamond into contact with a heated flat metal table under the oxidizing atmosphere at a high temperature. This chemical method uses no diamond powder.
FIG. 16 and FIG. 17 demonstrate conventional polishing apparatuses for general purposes. The object plate to be polished is hereinafter called a wafer now.
In FIG. 16, a polishing turn table (1) (rotary grindstone) is a diamond whetstone on which diamond powder is fixed by some means. An object wafer (2) is fixed on the bottom of a holder (3). The holder (3) is fixed to a shaft (4). The surface of the holder (3) is parallel with the face of the polishing turn table (1). An air-pressure cylinder or an oil pressure cylinder (5) is mounted on the top of the shaft (4) for pressing the holder (3) via the shaft (4) to the turn table (1). When the apparatus polishes a soft material, the load is unnecessary. But a heavy load is essential for grinding hard materials, i.e., diamond, c-BN or diamond-like carbon. An arm (6) supports the cylinder (5) and the shaft (4). The arm (6) can displace in a radial direction. The region of the whetstone table (1) which is in contact with the object moves in the radial direction for equalizing the defacement of the turn-table (1). The object wafer contacts the central part and the peripheral part of the polishing turn-table (1). The turn-table is worn out uniformly, maintaining the flatness of its face. The flatness of the turn-table ensures the long life time of the polishing table. In the apparatus of FIG. 16, the holder (3) does not rotate. The hard-material film of the wafer is whetted by the interaction between the revolving turn-table and the film.
When a hard-material having a Vickers hardness higher than Hv3000 is polished, the following problems arise.
One problem is the non-uniform polishing which is originated from the imparallelism of the wafer with the polishing table. The wafer must be kept rigorously in parallel with the grinding table in order to polish the whole surface uniformly. However, it is difficult to maintain the wafer in parallel with the polishing table. When the face of the wafer slants, the hard-material film is polished slantingly. Some parts of the film are thinner than other parts, which may be left unpolished. Therefore, the thickness of the film is not uniform in the whole plane. Such an uneven whetting is undesirable. It is important to obtain a uniformly thick film. In the case of a thin film, if the slanting polishing arises, the polishing starts from a corner which comes in contact with the polishing table at first, and then the other parts are later polished. Before some parts are polished at all, the base wafer is revealed under the film. Such a wafer is useless as a hard-material coated wafer. This problem is called a slanting polishing problem.
The other problem is originated when the wafer has an inherent wavy distortion, a concave distortion or a convex distortion. In the case of soft silicon, the convexes or concaves are entirely eliminated where the object is polished by a thickness larger than the heights of the convexes or concaves. However, such an easy solution does not apply to the hard-material coated wafers. The inherent distortion of wafers causes a serious problem in the case of hard-materials. Polishing diamond is far more difficult than silicon. The speed of polishing is far slower. In an initial stage of polishing, the turn-table comes into localized contact with the most prominent part of the film. The area of the contact is small enough, which allows polishing proceed in a pertinent speed. As the polishing operation progresses, the initially-concave parts come into contact with the polishing table. The increase in contact area reduces the pressure per unit area. Thus, the speed of polishing decreases. Eventually, the polishing ceases substantially. An addition to the load is needed to restore the progress of polishing.
However, the apparatus cannot impose an indefinitely heavy load upon the holder. The amount of the load is restricted within a range that will not cause the wafer to break. The limitation of the load causes a shortage of the pressure per unit area, which leaves unpolished parts or insufficiently-polished parts on the wafer. This is a defect of the static polishing apparatus shown in FIG. 16 which does not rotate the holder.
FIG. 17 indicates a perspective view of another prior apparatus which rotates the holder around its axis. The polishing turn-table (1) is a diamond whetstone. A wafer (2) is fixed to the bottom of a holder (3). The holder (3) is fixed to the shaft(4). Bearings sustain the shaft (4) vertical with respect to an arm (6), allowing the shaft (4) to rotate. An oil pressure cylinder or an air pressure cylinder (5) is mounted on the arm (6) above the shaft (4) for applying a load upon the holder (3) via the shaft (4). The arm (6) holds a motor (7) for driving the shaft (4). The rotation torque is transmitted from the motor (7) via a pulley, a belt and a pulley to the shaft (4). The torque rotates the holder (3) and the wafer (2). The wafer is polished by both the revolution of the turn-table (1) and the rotation of the holder (3).
This apparatus positively rotates the shaft (4) by a motor. However, a simpler example can be built by eliminating the motor and holding the shaft rotatably with bearings. Without the positive driving torque, the holder (3) rotates by itself around its own axis. Namely the line velocities of the contact regions are different between the central region and the peripheral region of the turn-table. The difference of the line velocities rotates the holder (3) in a certain direction at a moderate speed. This is a passive rotation. This rotation also is called xe2x80x9caccompanying rotationxe2x80x9d. The wafer (2) rotates on the turn-table whether the motor positively drives the shaft or the speed difference drives the wafer passively.
The rotation of the wafer equalizes the contact of the wafer on the turn-table. In both cases, the rotation of the wafer can solve the problem of the slanting polishing which is caused by an inclination of the shaft with respect to a normal of the turn-table.
However, the rotation of the wafer cannot solve the other problem of the unpolished parts being left on the wavy distorted, convex-distorted or concave-distorted wafer. If the distorted wafer were polished to the bottom of the convexes, a flat wafer could be obtained. However, the amount of polishing cannot be increased so much in the case of hard materials. An application of a heavy load is restricted in order to avoid breakage of the wafer. The limitation on the load compels the apparatus to terminate polishing far before the wafer is completely polished
The Inventors found that the prior polishing apparatus cannot polish a hard-material coated wafer with a distortion of a height more than 50 xcexcm perfectly, i.e., to the bottom of the convexes. Instead, wide unpolished parts are left on the wafer. Even for wafers of a distortion height between 20 xcexcm and 40 xcexcm, conventional polishing techniques are likely to leave some parts unpolished or imperfectly polished.
For facilitating an understanding of these problems, three kinds of imperfection of polishing are now clarified by referring to FIG. 21, FIG. 22 and FIG. 23. In FIG. 21 to FIG. 23, left figures indicate sectional views of complex wafers having a base wafer (substrate), and a rugged hard-material film, and right figures show the plan views after polishing. FIG. 21 denotes the case of an even wafer. The height of distortion is less than 5 xcexcm. Unpolished parts remain isolated at random within a circle distanced by 5 xcexcm to 10 xcexcm from the circumference. FIG. 22 indicates the case of a concave-distorted wafer. The height of distortion is, for example, 30 xcexcm. A continual, annular part is left unpolished in the middle region within an annular region extending between 5 xcexcm and 10 xcexcm from the circumference. Namely, the periphery and the central part are polished. FIG. 23 denotes the case of a convex-distorted wafer. The height of distortion is, for example, xe2x88x9220 xcexcm. A continual circumference part is left unwhetted. The polishing starts from the central part, develops to the middle region and then pervades to the periphery. This pattern is simpler and more promising than the other two displayed in FIGS. 21 and 22. These are the types of imperfections of polishing. The inclination polishing explained before by FIG. 16 is not shown here, because it appears in the case of a static holder and it is solved by adopting a rotary holder.
The applicability of the conventional wafer process is indispensable for the exploitation of hard-material coated complex wafers, that is, diamond-coated wafers, diamond-like carbon coated wafers or c-BN coated wafers to the fields of electronics, optics or optoelectronics. For example, the technology of photolithography must be able to be applied to the complex wafers for fine processing. Photolithography requires flatness of the object wafers. The various wafer processes have been highly developed in the silicon semiconductor technology. The hard-material coated complex wafers must cope with the conditions presented by the established wafer processes.
The hard-material coated complex wafer must comply with the requirements involved with the fabrication of semiconductor devices. In general, the fabrication technology demands a diameter of more than 2 inch and a thickness of less than 1 mm of a wafer.
The sizes of electronic devices become smaller year by year. The miniaturization of devices requires the reduction of thickness of wafers. The thinning technique has been established for silicon wafers.
Unlike silicon, wide, homogeneous wafers composed of only a single material cannot be produced pursuant to current technology in the case of hard materials, that is, diamond, diamond-like carbon and c-BN, because of the difficulties involved with making big and long single crystals. However, complex wafers can be produced by coating a pertinent substrate with a film of the hard-material. Instead of homogeneous wafers, two-component, complex wafers will be produced for the hard-materials. The non-homogeneous wafer consists of a substrate and a hard-material film. The hard-materials are hereinafter represented by diamond for brevity.
Diamond-coated wafers can be made by the known plasma CVD method, the hot filament CVD method and so forth. Various materials can be adopted as the substrates. The most convenient substrate is a silicon wafer, because the technology of making and processing silicon wafers has matured. It is easy to obtain flat silicon wafers at low cost.
A complex wafer can be made by depositing a diamond thin film on the substrate by the above-mentioned methods. The surface of the film is rugged. Then the rugged surface of the diamond-coated wafer must be polished into a smooth and flat surface.
However, diamond is the hardest material among all the materials obtainable at present. There is no material harder than diamond. Thus, diamond is mechanically polished by a polishing machine using diamond powder as whetting medium. In the polishing, high pressure must be applied to the surface of the object diamond. Thus, a diamond wafer must endure strong stress. However, a diamond wafer consists of a substrate wafer and a thin diamond film. The mechanical strength of the wafer is determined by the nature of the substrate. High loads affect mainly the substrate in the long run.
Silicon which is used as a substrate is a fragile material. When a silicon wafer is adopted as a substrate, the complex wafer is likely to break, in particular when the wafer has a big diameter and thin thickness. If another material is used as a substrate, the problem will not be solved, since the material is likely to be broken by the high pressure. This problem of high pressure must be solved in order to make a mirror diamond wafer.
There is still another problem. A strong inner stress arises in the complex wafer having a substrate and a film due to the two layered structure. A diamond is synthesized at a high temperature in vapor phase from the excited material gas. Then the wafer is cooled to room temperature. Thermal stress occurs in the complex wafer due to the differences of thermal expansion between the substrate and the film. In addition, a diamond film has inherently intrinsic stress. The thermal stress and the intrinsic stress distort the complex wafer convexly or concavely to a great extent.
Conventional polishing apparatuses whet a flat object by a flat holder and a flat polishing turn-table. The conventional machines are entirely unsuitable for polishing distorted objects. One alternative is polishing a distorted object by gluing a distorted object in a forcibly flattened state on a flat holder, pushing the object by the holder upon the turn-table, rotating the holder and revolving the turn-table in a conventional machine. However, such a superficial improvement would be in vain. One problem is the high probability of breakage of the wafers. Another problem is the difficulty of whetting the film uniformly. Another difficulty is a large fluctuation of the thickness of the film polished. These problems impede the application of the conventional polishing machines to the two-layered wafers having a substrate and a hard-material film.
One purpose of the present invention is to provide a broad complex wafer having a non-hard material substrate and a hard material film.
Another purpose of the invention is to provide an unbent complex wafer without inner stress.
Another purpose of the invention is to provide a smooth complex wafer without micro convexes or micro concaves.
Another purpose is to provide a method of polishing a complex wafer having a fragile substrate and a hard material film.
Another purpose is to provide a method of polishing a complex wafer without breaking or cracking the wafer.
Another purpose is to provide a method of polishing a complex wafer with distortion.
Another purpose is to provide a method of polishing a complex wafer without leaving unpolished parts.
Another purpose is to provide a method of polishing a complex wafer having a substrate and a hard film with little fluctuation of the thickness of the polished film.
Another purpose of the invention is to provide a method of polishing a hard film of a complex wafer with high efficiency.
Another purpose is to provide a machine for polishing a complex wafer having a non-hard material substrate and a hard material film into a mirror wafer.
Another purpose is to provide a machine for polishing a complex wafer without breaking or cracking the wafer.
Another purpose is to provide a machine for polishing a complex wafer with efficiency.
The hard material-coated wafer of this invention comprises a non-hard material base wafer and a hard material film with a thickness from 5 xcexcm to 100 xcexcm (preferably from 15 xcexcm to 50 xcexcm) and having a surface with a roughness less than Rmax50 nm and Ra20 nm, wherein the wafer bends convex on the side of the film with a bending height of 2 xcexcm to 150 xcexcm.
The method of producing a hard material-coated wafer comprises the steps of depositing a film of hard material of diamond, c-BN or diamond-like carbon to provide a thickness from 5 xcexcm to 100 xcexcm by a vapor phase deposition method, and polishing the film until the roughness attains Rmax less than 50 nm and Ra less than 20 nm.
The vapor phase method of producing the film can be selected from a filament CVD method, a microwave plasma CVD method, a radio-frequency plasma CVD method, a plasma flame method or so. The pertinent pressure of reaction is 1 Torr to 300 Torr. The material gas is hydrogen gas and hydrocarbon gas for the production of diamond or diamond-like carbon film. The material gas is hydrogen gas, boride gas and nitride gas for making a c-BN film.
In the formation of a diamond film or a diamond-like film, hydrogen gas and hydrocarbon gas constitute a main part of the material gas. However, the hydrogen gas can be replaced in whole or in part with a rare gas. Hydrocarbon gas can also be replaced with some organic gas or inorganic gas including carbon in the case of diamond or diamond-like carbon. It is preferable to dope the material gas with some organic gas or inorganic gas containing oxygen gas.
The vapor phase synthesis conditions should be selected in order to make a convex-distorted wafer which bends toward the side of the synthesized film. This is an important matter. A flat wafer is surprisingly rather inoperative, since a substantially flat wafer has a wavy surface unsuitable for polishing. A concave-distorted wafer is also improper. Only convex-distorted wafers can be polished uniformity by the polishing apparatus of this invention. The film of the convex-distorted wafer should be polished until the roughness of the film is reduced to Rmax less than 50 nm and Ra less than 20 nm. Although the surface is not perfectly smooth, a film having such a roughness is suitable for forming electrodes, implanting impurity atoms, diffusing impurities or etching metals, oxides or semiconductors selectively by photolithography.
FIG. 1 shows a sectional view of a hard material-coated wafer of the present invention. This is a convex-distorted wafer.
Partly because of the difference of thermal expansion coefficients and partly because of the big thickness of the film, strong inner stress occurs in the complex wafer when the wafer is cooled to room temperature after the synthesis of the hard material film. The strong inner stress deforms the complex wafer either to a convex-form or to a concave-form. The distortion is now expressed by the height H, which is measured from the center of an imaginary plane connecting opposing peripheral portions of the wafer to the bottom surface of the wafer. The sign of the height H is determined herein to be positive for concave distortion and to be negative for convex distortion. This invention prefers convex-distorted wafers with negative heights H between xe2x88x922 xcexcm and xe2x88x92150 xcexcm. FIG. 1 shows a convex-distorted wafer of a negative distortion height which is suitable for the polishing apparatus of the present invention.
The Inventors have found that the distortion of wafers can be controlled by controlling the conditions of the vapor phase deposition of the film on the substrate. Some synthesis conditions can make a flat wafer. A flat wafer seems to be preferred for polishing. However, this is wrong. A flat wafer actually contains waves, as shown in FIG. 2. The wafer which seems flat is actually partly concave and partly convex. Namely, a flat wafer is more complex than a convex wafer or a concave wafer as a whole. When a flat wafer with a wavy surface is polished, unpolished portions remain. Thus this invention avoids flat wafers. Further, this invention does not employ concave-distorted wafers because of the difficulty of polishing.
The present invention requires the following conditions of hard materials, substrates and distortion as optima.
[A. hard material films]
Diamond films, diamond-like films and c-BN films should be provided with the conditions below:
1. Thickness of film: 5 xcexcm to 100 xcexcm; preferably 15 xcexcm to 50 xcexcm. A large film thickness raises the cost of formation of the film. A 1000 xcexcm thick film is operative but such a thick film needs too much material and too long time for deposition. Thus, a film thicker than 100 xcexcm is disadvantageous from the economical standpoint. On the other hand, too small of a film thickness complicates polishing. The base wafer is exposed at some parts of the surface after polishing. Sometimes the wafer is broken by the contact of the base material to the polishing plate. Therefore, this invention employs films with a thickness more than 5 xcexcm. Preferably, the thickness is 15 xcexcm to 50 xcexcm.
2. Roughness of surface: Rmax is less than 50 nm. Ra is less than 20 nm.
If the surface roughness is larger than these values, the wafer cannot be utilized as a wafer for electronic devices or a wafer for abrasion resistant tools. If the roughness of a wafer is large, the photolithograpy techniques are not suitable to form fine patterns of wires of devices on the wafer. A high roughness raises the friction coefficient of the complex wafer. The wafer with high friction is not suitable as a material of abrasion resistant tools.
[B. base wafer (substrate)]
The base wafer on which a hard material film is grown is a non-hard material which is suitable for being formed into a wide, thin plate. The material of the base wafer is one of Si, GaAs, GaP, AlN, SiC, Si3N4, LiTaO3, LiNbO3, Li2B4O7, PbTiO3, PZT(PbZrO3xe2x80x94PbTiO3) or quartz. A Si single crystal wafer is, in particular, the most desirable. Furthermore, a (100) Si wafer is the optimum material among various oriented silicon wafers.
The optimum thickness depends on the material selected for the base wafer. In general, the thickness of the base wafer should be 0.1 mm to 1 mm. A base wafer thinner than 0.1 mm would undergo a large distortion and be prone to breakage. On the other hand, a wafer thicker than 1 mm cannot be treated by the wafer process. Thus, devices cannot be built on such a thick wafer by the process. If devices are made on the thick wafer, the devices cannot be mounted on packages unless the wafer is thinned by grinding its bottom surface.
With regard to the shape, circular wafers are the most convenient for handling, e.g., conveyance, fixation, storing, or supporting. However, a rectangular wafer or square wafer is also suitable as a base wafer. Such wafers are used for special purposes. In general, circular wafers are most suitable for treatment by the common semiconductor process like silicon wafers.
The diameter is an arbitrary parameter which will be determined in accordance with the application for which the wafer is designed. The diameter, however, must be larger than one inch (25 mm) to enhance the efficiency of the treatment in the wafer process. 2-inch wafers, 3-inch wafers, 4-inch wafers, 5-inch wafers or 8-inch wafers are also useful as base wafers of the invention.
[C. distortion]
This invention requires that, after the synthesis of the films, the wafer exhibit a convex distortion. Namely the wafer should be distorted convex to the side of the film monotonously from the periphery to the center. The absolute value |H| of the height of distortion must range from 2 xcexcm to 150 xcexcm. The distortion of the wafer is here expressed by the height H measured from the center of an imaginary plane including the peripheral circle of the wafer to the lower surface of the wafer. The distortion may be designated by a curvature radius R. R and H are simply related by an equation R=D2/8H, where D is the diameter of the wafer. Further, the convex-distortion is allocated with a negative H but the concave-distortion is assigned with a positive H. Thus the requirement of distortion is briefly described by xe2x88x92150 xcexcmxe2x89xa6Hxe2x89xa6xe2x88x922 xcexcm, and preferably by xe2x88x9250 xcexcmxe2x89xa6Hxe2x89xa6xe2x88x925 xcexcm.
The present invention is not specifically directed to a wafer without distortion, that is, H=0. Wafers having no distortion may seem to be the best for the following polishing process. However, if a wafer has no macroscopic distortion, it actually has some waves which invite more complex modes of bending, as shown in FIG. 2. Such a wavy wafer cannot be polished perfectly by any technique. Unpolished parts or insufficiently polished parts remain at random on the surface. Wafers exhibiting some bending are more suitable for polishing, even though the distortion is pretty large. The minimum distortion is determined to be 2 xcexcm for excluding non-distortion H=0. On the other hand, the maximum distortion is 150 xcexcm. A wafer having a distortion height larger than 150 xcexcm will exhibit some unpolished regions. Wafers should be uniformly polished overall. The uniform polishing is impossible for wafers which bend more than 150 xcexcm. Thus this invention attempts to avoid a distortion of more than 150 xcexcm.
Next, the polishing method and polishing machine of the present invention will be explained.
The polishing method of the present invention comprises the steps of attaching a complex wafer having a substrate and a hard film of more than Hv3000 to a center of a bottom face of a holder in a convex-distorted state, introducing the convex film to a rotating turn-table, applying a heavy load on the center of the holder, rotating the holder, inclining the bottom face of the holder with respect to the turn-table, bringing a contact region between the wafer and the turn-table from the center to the peripheral portion of the wafer gradually, and thereby moving a polishing region from the center to the periphery of the film. Since the contact region moves from the center to the periphery, the entire surface is perfectly polished, leaving no unpolished portions. The movement of rotation around a slanting axis whose direction is changing is now called a xe2x80x9cprecessionxe2x80x9d or a xe2x80x9cswaying movementxe2x80x9d. In the restrict meaning in physics, the precession denotes the rotation in a rigorous cone of the slanting axis itself. However, the word xe2x80x9cprecessionxe2x80x9d here means arbitrary rotation movement of the slanting axis of the holder. The locus of the axis includes not only circles but also ellipsoids. The swaying movement means the reciprocal inclinations of the rotational axis in the radial direction. The locus of the axis is a part of a straight line. Thus, swaying movement is a limit of the precession whose locus ellipsoid has a shorter axis of a length of zero. Thus, sometimes both the precession and the swaying movement are represented by precession in short.
This invention polishes a hard material coated wafer by attaching the wafer to the center of the bottom of the holder, contacting the holder on a whetting turn-table, rotating the holder and allowing the holder take a precession or a swaying movement. In addition, the invention preferably reciprocates the holder in the radial direction. Thus, the present invention can mirror-polish the whole surface of the wafer into a flat film of uniform thickness without breaking the wafer. No portion is left unpolished. All the surface is whetted uniformly by the precession or the swaying movement.
Further, it is more desirable to insert a buffer between the wafer and the holder. The buffer allows the wafer to deform, absorbs the impulse and prevents the wafer from breaking. The buffer can be made from an elastic material, for example, rubber, plastics or so.
This invention can be applied to a convex-distorted wafer, a substantially flat wafer and a concave-distorted wafer. In each instance, the wafer must be attached to the holder in the state of the convex-distortion. This restriction is important for polishing. The pertinent height of the convex-distortion ranges from 3 xcexcm to 50 xcexcm in the wafer.
In the case of a convex-distorted wafer with a pertinent curvature, the wafer can be attached to the buffer which is connected to the bottom of the holder. The wafer is held by the holder via the buffer. The elastic deformation of the buffer maintains the suitable convex-distortion of the wafer.
Otherwise, when the wafer is flat, concave-distorted or convex-distorted with a curvature out of the optimum range between 3*m and 50 xcexcm, a spacer is inserted between the wafer and the buffer for deforming the wafer forcibly into the convex-distorted state so as to take on a distortion height within the range of 3 xcexcm and 50 xcexcm. The use of the spacer is inevitable in the case of the wafer whose inherent distortion is outside of the range between xe2x88x923 xcexcm and xe2x88x9250 xcexcm. But the adoption of the spacer is optional in the case of the wafer having an inherent distortion from xe2x88x923 xcexcm to xe2x88x9250 xcexcm.
What is important is the precession or the swaying motion of the holder for bringing the whole convex-distorted surface into contact with the polishing turn-table. The amplitude of the precession or the swaying motion is nearly equal to the center angle of the curved face of the convex-distortion of the wafer. The precession of the holder is the most essential part of the invention. Without the precession, nothing but the central part would come in contact with the polishing table, and the peripheral portions could not be polished at all. The precession enables the machine to whet the periphery of the wafer.
As explained before, the optimum height of distortion is xe2x88x923 xcexcm to xe2x88x9250 xcexcm. The height of distortion is defined as a distance between the top of distortion and the imaginary plane connecting opposing peripheral portions of the wafer.
This invention can be applied to any size wafer. In particular, the method is suitable for a wafer having a diameter of more than 1 inch (25 mm) and a thickness between 0.1 mm and 2.1 mm. The wider the wafer is, the more difficult the polishing becomes. However, this application can be applied to wafers having a diameter of more than 1 inch (25 mm). The method can treat 2-inch wafers, 3-inch wafers and 4-inch wafers.
The distortion of wafers is here represented by the height(H). H is determined to be positive for concave distortions. H is negative for convex distortions. The curvature of wafers differs for the same distortion height if the diameters of the wafers are different. In the case of a simple quadratic distortion, the radius R of curvature is related to the distortion height H by the equation H=D2/8R, where D is the diameter of the wafer. Namely, the curvature radius R is in reverse proportion to the distortion height. The product of the two parameters is D2/8. The curvature 1/R is of course in proportion to the distortion height H.
Irrespective of the initial distortion, the wafer is attached to the bottom of the holder via the spacer which gives the wafer a pertinent convex-distortion of xe2x88x923 xcexcm to xe2x88x9250 xcexcm. Thus, the wafer is connected to the holder in the convex-distorted state of H=xe2x88x923 xcexcm to xe2x88x9250 xcexcm. If the wafer has the suitable distortion of H=xe2x88x923 xcexcm to xe2x88x9250 xcexcm, the wafer can be easily attached to the spacer and fixed on the holder. However, in many cases, the curvature of the spacer differs from the inherent curvature of the wafer. Nevertheless, such wafers can still be polished by the apparatus of this invention. For example, even an originally concave-deformed wafer can be forcibly bent into the reverse tendency and attached to the holder. The difference of the curvatures gives rise to inner stressing the sample. However, the buffer protects the wafer from breaking.
A convex-distorted wafer can be attached to the holder via a convex spacer and a buffer without forcibly bending the wafer. A flat wafer and a concave-distorted wafer also are fixed to the holder via a convex spacer and a buffer.
In preparation, the wafer is polished by a polishing machine which applies the precession or the swaying motion to the holder. The machine can polish the wafer until the surface maximum roughness Rmax is reduced to less than 50 nm and the average roughness Ra is reduced to less than 20 nm on at least 50% of the whole surface. Here Rmaxxe2x89xa650 xcexcm and Raxe2x89xa620 xcexcm define smooth enough surfaces to enable the wafers to be treated by the wafer processes, including photolithography. Accordingly, the apparatus of the present invention can whet a hard-material coated wafers to a smooth wafer having more than half of its surface polished. If an unpolished portion remains, it is in a peripheral circular zone. In many cases, the mirror-polished portion exceeds 90% of the whole surface. The ratio of the mirror-polished portion can be raised to 100% by the adjustment of the amplitude of the precession angle or the swaying angle. Since this invention extends the contact region of the convex-distorted wafer from the center to the periphery gradually, an arbitrary portion from the center can be polished continually by adjusting the angle of the precession. This invention prevents such a random distribution of unpolished portions shown in FIG. 21 or a middle zone of unpolished part denoted in FIG. 22 or intermediate polishing shown in FIG. 23.
In addition of the precession or the swaying motion, it is also profitable to displace the holder in a direction not parallel with the circular direction. Since the turn-table is rotating, the non-circular displacement induces an effective radial movement of the contact portion. Thus the motion in the non-circular direction of the holder can be called now a radial displacement of the holder. The radial displacement of the holder changes the contact region of the turn-table and equalizes the defacement of the turn-table in a wide range. Reciprocal displacement is more preferable for dispersing the defacement of the turn-table. If the defacement occurs uniformly on the turn-table, the flatness of the turn-table is maintained for a long time. Namely, the uniform defacement maintains the flatness and the parallelism of the surface of the polishing turn-table, and prolongs the life time of the turn-table. The reciprocal movement or the displacement in the radial direction is shown in FIG. 30 and FIG. 31. A solid circular line indicates an outer position (S) of the holder. xcexa3 s denotes the locus of the contact region of the holder(wafer) at S on the turn-table. A dotted circular line designates an inner position (T) of the same holder. xcexa3 t is the locus of the contact region of the holder (wafer) at T on the turn-table. When the holder is displaced or reciprocated between S and T, the whole zone between a small circle m and a big circle p comes into contact with the wafer.
The initial thickness of the film is arbitrary. The film thickness should be determined not to induce too large deformation of the wafer. The thickness of the substrate is also arbitrary. If the thickness of the substrate is determined to be, for example, 0.1 mm to 2 mm for an economical reason, the suitable initial thickness of the film shall be 0.01 mm to 1 mm.
It is difficult to make a thick hard film from the standpoint of economy of time and material. The practical thickness of the film is less than 200 xcexcm perhaps. However, this invention is applicable to any thickness of films of course. The thicker the film is, the smaller the fluctuation of the thickness after whetting.
This invention is capable of reducing the fluctuation of the film thickness of the smooth parts having a roughness less than Rmax50 nm and Ra20 nm within xc2x110%. Two methods are available for suppressing the fluctuation of the film thickness of the smooth parts with Rmax less than 50 nm and Ra less than 20 nm within xc2x110%.
(A) A distortion-reducing method which gradually reduces the distortion according to the progress of hetting. FIG. 26 and FIG. 27 demonstrate this method. The wafer (shown in an inverted state) has a structure of a (substrate 100)/(hard film 102a)/(hard film 102b). The top hard film 102b has compressive stress. When the film 102b is removed, the excess compressive stress lessens and the distortion is reduced.
(B) A two-layered method which coats the substrate first with a harder layer and further coats the harder layer with a softer layer. FIG. 28 and FIG. 29 show the method. The structure is a (substrate 100)/(hard film difficult for polishing 102c)/(hard film facile for polishing 102d). When almost all the top film is polished away, the film difficult for polishing is revealed. The thickness of the final film is nearly equal to the thickness of the film difficult for polishing.
It takes a long time to polish a hard material film. This invention is capable of enhancing the whetting speed and raising the throughput of whetting by the following improvement. The improvement makes the contact region discontinuous for reducing the area of the substantial contact regions. There are some ways for reducing the contact area.
One is forming lattice-like grooves on the surface of the whetstone turn-table, as shown in FIG. 32. Many lengthwise grooves and many crosswise grooves are formed on the turn-table. Many small squares will be in contact with the wafer. Thus, the substantial area of contact regions is reduced by the grooves. The reduction of the contact area heightens the polishing speed by enhancing the pressure per unit area.
Another way for reducing the contact area is forming circular grooves on the turn-table. FIG. 33 indicates the plan view of the turn-table having concentric circular grooves.
Another way is forming lattice grooves or concentric circular grooves on the hard film. The grooves are not used as hard material coated chips. This way is applicable for the wafer which requires small chips for devices.
The slanting polishing of this invention polishes hard-material coated wafers by free diamond powder or fixed diamond powder. In the case of using free whetting powder, a polishing liquid including diamond powder is supplied on the turn-table which has a metal surface or a cloth surface.
In the case of using fixed powder, the turn-table must be a diamond whetstone itself. Diamond whetstones are classified into several kinds by the methods of fixing diamond powder on the base plate. This method can adopt any one of the resin-bonded diamond whetstones, metal-bonded whetstones, electrodeposited diamond whetstones, small diamond pellet whetstones and diamond pellet whetstones. These whetstones polish an object by physical action. In addition to the diamond whetstones, a flat metal turn-table heated at a suitable temperature can polish a hard material coated film by thermochemical reaction.
The advantages of the invention are now explained.
Bulk crystals of diamond and c-BN have been synthesized by some methods. All the bulk crystals had a poor practical significance aside from the academic meanings, because they had too narrow of a surface area. This invention succeeds in making big-sized wafers of the hardest materials,i.e. diamond, diamond-like carbon and c-BN, for the first time. The hard material wafers have a complex structure which includes a commonplace substrate and a hard film deposited on the substrate. The wafers are different from Si wafers or GaAs wafers which are homogeneous and entirely composed of a single material. The complex wafers are quite useful as a material for making electronic devices, since such devices make the best use of only the surface of the wafers.
The chemical vapor phase deposition (CVD) enables this invention to make large sized complex wafers on wide substrate wafers. Supply of large-sized wafers reduces the cost of producing devices by the wafer process.
The wafers of the present invention, however, are deformed by the difference of the thermal expansion between the substrate and the film. Conventional polishing machines are commonly understood as being effective only to polish flat objects but are entirely ineffective to polish distorted objects.
The Inventors disagree with this common understanding. A flat wafer is rather difficult to polish. Convex-distorted wafers with a distortion ranging from xe2x88x92150 xcexcm to xe2x88x922 xcexcm can uniformly be polished by a polishing machine with a holder which rotates with swaying motion or precession motion. Concave-distorted wafers can be also polished by inserting a convex spacer between the buffer and the wafer.
One of the important matters of this invention is the discovery of the possibility of polishing of deformed objects, which allows this invention to make mirror wafers of hard material for the first time. The machine which this invention adopts polishes a convex-distorted wafer from the center to the periphery or vice versa by inclining the holder gradually until the entire convex surface is fully polished to a roughness less than Rmax50 nm and Ra20 nm, which enables photolithograpy to make various devices on the hard-material wafer.
A hard-material coated wafer is likely to bend in a convex shape or in a concave shape because of the difference of thermal expansion coefficients between the substrate and the hard-material film. Even when a complex wafer is immune from the distortion as a whole, the wafer has waves. In any cases, complex wafers are not flat in a rigorous meaning. Conventional machines could not polish all the surfaces without leaving unpolished portions.
This invention succeeds in polishing the whole surface of a complex wafer without leaving unpolished parts by attaching the wafer in convex-distortion to a holder, rotating the holder on a rotary turn-table and giving the holder a swaying motion or a precession which enables all the parts on the surface to come into contact with the turn-table. The polished part has a sufficient smoothness of less than Rmax50 nm and Ra20 nm. The present invention can suppress the fluctuation of the film thickness within xc2x110%.
A two-layered version film is also proposed for reducing the fluctuation of thickness of the polished film. This version coats the substrate with a polishing-difficult layer first until a definite thickness on the polishing difficult layer is formed and then deposits a polishing-easy layer. When the two-layered wafer is polished, the softer top layer is perfectly polished and the harder undercoat is revealed overall. Thus. the distribution of the thickness becomes uniform in the whole of the surface.
Big bulk crystals of hard material, that is, diamond, diamond-like carbon and c-BN cannot be grown by the present state of technology. Films of the hard materials can be synthesized by the vapor phase deposition methods. However, it takes a very long time and large financial expenditures to make a homogeneous wafer which contains no other material than the object hard material by growing a thick hard-material film on a substrate and eliminating the substrate. On the other hand, a complex wafer having a thin hard-material film is sufficient as a material for applications such as semiconductor devices, optical devices or optoelectronic devices. The complex wafers must be mirror-polished for the convenience of photolithography or other wafer processes. However, complex wafers are accompanied by strong distortion due to large inner stress. This invention enables mirror polishing of the distorted complex wafers for the first time. This is an important invention which leads to effective applications of the hard materials to various fields of technology.